1. Field of the Invention
The present invention relates to a device for treating exhaust gas containing soot particles, in particular with a so-called electrostatic filter or electric filter, as well as a suitable method for converting soot particles of an exhaust gas. The invention is used, in particular, in the treatment of exhaust gases of mobile internal combustion engines in the field of automobiles, in particular in the treatment of exhaust gases resulting from diesel fuel.
A multiplicity of different concepts for eliminating soot particles from exhaust gases of mobile internal combustion engines have already been discussed. In addition to wall-flow filters which are alternately closed, open secondary flow filters and gravity precipitators, etc., systems have already been proposed in which the particles in the exhaust gas are charged electrically and then deposited by using electrostatic attraction forces. Those systems are known, in particular, by the term “electrostatic filter” or “electric filter.”
Generally (a plurality of) discharge electrodes and collector electrodes, positioned in the exhaust line, are proposed for such electric filters. In that context, for example, a central discharge electrode which runs approximately centrally through the exhaust line and a surrounding lateral surface of the exhaust line as a collector electrode are used to form a capacitor. Through the use of that configuration of the discharge electrode and the collector electrode, an electrical field is formed transversely with respect to the direction of flow of the exhaust gas, wherein the discharge electrode can be operated, for example, with a high voltage which is in the range of approximately 15 kV. As a result, in particular corona discharges can be formed through which the particles which flow through the electrical field with the exhaust gas are charged in a unipolar fashion. As a result of that charging, the particles migrate to the collector electrode due to electrostatic Coulomb forces.
In addition to systems in which the exhaust line is embodied as a collector electrode, systems are also known in which the collector electrode is embodied, for example, as a wire mesh. In that context, the accumulation of particles on the wire mesh serves the purpose of combining the particles, where appropriate, with further particles in order to therefore form an agglomeration. The exhaust gas which flows through the mesh or grid then carries the relatively large particles along with it again and carries them to conventional filter systems.
Even if the systems described above have heretofore proven suitable for the treatment of soot particles, at least in trials, the implementation of that concept for series production in motor vehicles presents serious challenges. That applies, in particular, with respect to the greatly fluctuating, at times very heavy, soot load in the exhaust gas, as well as the desired retrofitability of such a system for currently existing exhaust systems. In addition, it is necessary to take into account the fact that the improved performance of such exhaust systems in terms of the elimination of soot particles also makes it necessary to perform (periodic or continuous) regeneration of the filter systems, involving the soot being converted into gaseous components.
With respect to the regeneration of filter systems, it is also known, in addition to the intermittent regeneration by brief heating, that is to say burning off of the soot (catalytically motivated oxidative conversion), to convert soot through the use of nitrogen dioxide (NO2). The advantage of continuous regeneration with nitrogen dioxide is that the conversion of soot can already take place in that case at significantly lower temperatures (in particular lower than 250° C.). For that reason, continuous regeneration is preferred in many applications. However, that leads to the problem that it is necessary to ensure that the nitrogen dioxide in the stream of exhaust gas comes into contact to a sufficient degree with the deposited soot particles.
In that context, there are also technical difficulties in the implementation of continuous operation of such exhaust systems in motor vehicles, wherein the different loadings of the internal combustion engines lead to different streams of exhaust gas, compositions of exhaust gas and temperatures.